1. Field of the Invention
The present invention relates to a mixer which is used in a frequency converter, a modulator or the like.
2. Description of the Related Art
The conventional mixer as shown in FIG. 3 is called the Gilbert mixer and widely used in frequency converters and the like. As illustrated in FIG. 3, a first differential amplifier circuit 21 and a second differential amplifier circuit 22 are connected in a balanced way by a third differential amplifier circuit 23. Specifically, the emitters of a pair of transistors 21a and 21b which constitute the first differential amplifier circuit 21 are connected with each other and also connected with the collector of one transistor 23a in a pair of transistors which constitute the third differential amplifier circuit 23; and the emitters of a pair of transistors 22a and 22b which constitute the second differential amplifier circuit 22 are connected with each other and also connected with the collector of the other transistor 23b in the pair of transistors which constitute the third differential amplifier circuit 23. The emitters of the transistors 23a and 23b in the third differential amplifier circuit 23 are connected with each other and also connected with a constant current source 24.
The collector of the one transistor 21a in the first differential amplifier circuit 21 and the collector of the one transistor 22a in the second differential amplifier circuit 22 are connected with one balanced output terminal 25 and supply voltage B is fed to them through a common load resistor 26. Also, the collector of the other transistor 21b in the first differential amplifier circuit 21 and the collector of the other transistor 22b in the second differential amplifier circuit 22 are connected with the other balanced output terminal 28 and supply voltage B is fed to them through a common load resistor 29.
The base of the one transistor 21a in the first differential amplifier circuit 21 and the base of the other transistor 22b in the second differential amplifier circuit 22 are connected with each other and also with one balanced input terminal 30. The base of the other transistor 21b in the first differential amplifier circuit 21 and the base of the one transistor 22a in the second differential amplifier circuit 22 are connected with each other and also with the other balanced input terminal 31.
Also, the bases of the transistors 23a and 23b in the third differential amplifier circuit 23 are respectively connected with the first balanced input terminals 32 and 33.
In the above configuration, a first signal, for example a high frequency signal, is inputted between the first balanced input terminals 32 and 33 in a balanced way; and a second signal, for example a local oscillation signal, is inputted between the second balanced input terminals 30 and 31. Consequently, an intermediate frequency signal as a result of frequency conversion is outputted between the balanced output terminals 25 and 28. A tuning circuit (not shown) is connected between the balanced output terminals 25 and 28.
The local oscillation signal as the first signal is supplied, for example, from a balanced type oscillation circuit as illustrated in FIG. 4. In this circuit, a pair of oscillation transistors 41 and 42 share one resonant circuit 43 which is connected between their respective collectors and bases, and a local oscillation signal is outputted between the collectors in a balanced way.
In the above conventional mixer, supply voltage is serially fed to the first and second differential amplifier circuits 21 and 22 and the third differential amplifier circuit 23 and therefore the voltage applied to each differential amplifier circuit is about one half the supply voltage. Accordingly, this voltage drop may cause mixer performance deterioration.
Most mixers use integrated circuits (IC). The latest type integrated circuits are designed to operate at low voltage, so the influence of supply voltage on the mixer performance is considerable.
Furthermore, the conventional mixer requires peripheral circuits such as oscillation circuits and resonant circuits, which makes the overall structure of the mixer complicated.
The present invention has been made in view of the above circumstances and provides a mixer which assures satisfactory performance even at low supply voltage and has a simple overall structure including peripheral circuits.
In order to solve the above problem, according to one aspect of the present invention, the mixer comprises: first and second transistors which constitute a balanced oscillation circuit with their respective emitters connected with each other; a third transistor whose emitter is connected with the emitter of the first transistor; and a fourth transistor whose emitter is connected with the emitter of the second transistor, wherein a first signal is inputted between a base of the third transistor and a base of the fourth transistor in a balanced way; and the third transistor and the fourth transistor mix an oscillation signal with the first signal and a second signal is outputted from the third transistor and the fourth transistor in a balanced way.
According to another aspect of the invention, the mixer has a first parallel resonant circuit which determines an oscillation frequency and a second parallel resonant circuit which resonates with the second signal; the first parallel resonant circuit is connected between a collector and the base of each of the first and second transistors; and the second parallel resonant circuit is connected between a collector of the third transistor and a collector of the fourth transistor.
According to another aspect of the invention, the first parallel resonant circuit is made up of a first capacitor, and first and second inductors which are serially connected with each other and connected in parallel with the first capacitor; and a point of connection between the first capacitor and the first inductor is connected with the collector of the first transistor; a point of connection between the first capacitor and the second inductor is connected with the collector of the second transistor. Also, the second parallel resonant circuit is made up of a second capacitor, and third and fourth inductors which are serially connected with each other and connected in parallel with the second capacitor; a point of connection between the second capacitor and the third inductor is connected with the collector of the third transistor; a point of connection between the second capacitor and the fourth inductor is connected with the collector of the fourth transistor. Here, supply voltage is fed to a point of connection between the first inductor and the second inductor and a point of connection between the third inductor and the fourth inductor.
According to another aspect of the invention, the mixer includes a composite resonant circuit which has parallel resonance frequencies in the respective vicinities of an oscillation frequency and the second signal, wherein the collector of the first transistor and the collector of the third transistor are connected with each other and the collector of the second transistor and the collector of the fourth transistor are connected with each other. The composite resonant circuit is connected between the collectors of the first and third transistors and the base of the second transistor as well as between the collectors of the second and fourth transistors and the base of the first transistor.
According to another aspect of the invention, the composite resonant circuit includes a first capacitor; a serial circuit which has a second capacitor, and first and second inductors and is connected in parallel with the first capacitor with the second capacitor located between the first and second inductors; and third and fourth inductors which are serially connected with each other and connected in parallel with the second capacitor. The first and second capacitors and the first and second inductors generate parallel resonance in the vicinity of the oscillation frequency and the second capacitor and the third and fourth inductors generate parallel resonance in the vicinity of the second signal frequency. The point of connection between the first capacitor and the first inductor is connected with the collectors of the first and third transistors and the point of connection between the first capacitor and the second inductor is connected with the collectors of the second and fourth transistors. Here, supply voltage is fed to the point of connection between the third and fourth inductors.
According to another aspect of the invention, the second signal is outputted from both ends of the second capacitor.